Before the onset of speech, an index of the developing communicative significance of infant social actions is their temporal coordination. One type of temporal coordination occurs when an action (e.g., a vocalization) both begins and ends during the course of another (e.g., a gaze at mother). This pattern of temporal embedding suggests the two actions are part of a single expressive event. The proposed project will use a new software simulation bootstrapping procedure to document embedding and other patterns in the temporal coordination of infant social actions. 50 infants will be observed longitudinally at 3, 6, and 9 months interacting with their mothers in a face-to-face still- face procedure. Descriptive coding will be used to categorize infant vocalizations and facial expressions by affective tone. Direction of infant gaze will be coded as at mother's face/eyes or away. Patterns of reciprocal embedding are hypothesized for actions in all pairs of behavioral modalities. Infants are expected, for example, to begin and end facial expressions in the course of a gaze at mother's face; but they are also expected to begin and end gazes at mother in the course of a facial expression. Software incorporating a bootstrapping procedure simulates the temporal progression of infant actions independently within each modality, creating multiple simulated sessions with random levels of between modality coordination. It will indicate whether infants reciprocally embed social actions at greater than chance levels. Increases in temporal embedding are expected with age as infants become more competent communicators. Decreases in embedding are expected when infant's are frustrated by maternal non-responsiveness in the still face procedure. The relative strength of embedded patterns involving effectively positive and negative actions will be contrasted to shed light on their respective expressive clarity. In sum, a new approach will be used to determine whether embedded events occur at greater than chance levels 1) overall, 2) during interaction rather than a maternal still-face, 3) at different ages, and 4) among different effectively toned pairs of social actions. A long term objective of the project is to gather pilot data for future investigations of temporal coordination between infant and mother (e.g., turn-taking). This will open the way for exploration of coordination deficits stemming from infant communicative disorders such as autism and Down syndrome, as well as coordination deficits stemming from maternal risk factors such as depression.